The authors say an ecosystem's economic value can be measured in terms of the goods and services it provides - climate regulation, for example, water filtration, soil formation, and sustainably harvested plants and animals.

Pricing them is difficult, since many of the goods and services are not bought and sold in the conventional economy.

Short-sighted exploiters

So economists assign values to these "non-marketed services" in different ways, perhaps according to the cost of replacing them, or assessing how much people would be willing to pay for them.

The authors looked at five converted ecosystems:

a Malaysian tropical forest razed for intensive logging

a tropical forest in Cameroon destroyed for small-scale agriculture and commercial plantations

a mangrove system in Thailand turned over to shrimp farming

a Canadian marsh drained for agriculture

a coral reef in the Philippines dynamited for fishing.

Overall, they say, roughly half of an ecosystem's total economic value is lost when it is converted from its wild state to human use.

They found that in each of the five case studies the loss of services such as storm and flood protection, atmospheric carbon sinks, hunting and tourism outweighed the marketed benefits that came with conversion.

One-third of the world's wild nature has been lost since I was a child and first heard the word 'conservation'. That's what keeps me awake at night

Andrew Balmford

The total value of the ecosystems before they were converted, the researchers concluded, ranged from 14% to nearly 75% higher than their values after conversion.

They say a global nature reserve network would guarantee goods and services worth annually at least $4,400bn more than the profits to be made after conversion - a 100:1 benefit-cost ratio.

Dr Paul Jefferiss of the RSPB said: "Less than one-sixteenth of the annual defence budget would protect the Earth's nature in perpetuity."

Distorted reality

The reasons why people so often ignore the net benefit of conservation, they say, include lack of information, the failure of markets to "capture" and value the services provided, and tax incentives and subsidies which encourage conversion.

Ways of putting a market price on ecosystem services include carbon taxes and credits, higher prices for eco-friendly products, and direct payments to local communities in significant conservation areas.

One of the authors, Andrew Balmford, of the University of Cambridge, UK, said: "In terms of immediate 'bang for buck', directly challenging subsidy schemes is a good way to improve both economic efficiency and the environment.

"The economics are absolutely stark. We thought that the numbers would favour conservation, but not by this much."

The authors say natural systems are changing from their intact state at 1.2% annually, or 11.4% since the Rio de Janeiro Earth Summit in 1992.

Suspect accounting

Andrew Balmford said: "People are hearing a message that nature is being eroded, but it takes a while to sink in, even for me.

"One-third of the world's wild nature has been lost since I was a child and first heard the word 'conservation'. That's what keeps me awake at night."

Another author, Professor Robert Costanza, of the University of Vermont, US, said: "Enron and other companies got into trouble through bad accounting.

"We've been doing exactly the same on the global scale, counting the destruction of nature as revenue."